Tire pressure maintenance system

ABSTRACT

A gas transfer system is provided for establishing or maintaining a predetermined gas pressure within a plenum, such as a tire. In an exemplary embodiment, the gas transfer system includes a power source, a pressure sensor, a control unit, and a gas transfer mechanism. Preferably, a gas transfer mechanism includes a micromechanical device, comprising one or more pumping units that transfer gas from one pressure zone to another. In one embodiment, pumping is accomplished, in part, by heating the gas within a sealable chamber of a pumping unit to cause the pressure of the gas to increase. In another embodiment, the change in pressure of the gas caused by compression of a tire provides a pumping force. Valves are provided for regulating movement of the gas through the gas transfer mechanism and can include electromechanical valves responsive to signals from the control unit, or passively biased valves responsive to applied gas pressure.

This application is a division of U.S. application Ser. No. 08/191,315,filed Feb. 1, 1994 now U.S. Pat. No. 5,472,032.

FIELD OF THE INVENTION

The present invention relates to a gas transfer system, and moreparticularly to apparatus for establishing and/or maintaining apredetermined gas pressure within a tire or other plenum.

BACKGROUND OF THE INVENTION

Proper inflation of automotive or other tires currently relies on nearperfect sealing of tires to tire rims and excellent performance of tirevalves, together with periodic monitoring of tire pressure and re-fillas necessary. Improper tire inflation causes reduced gasoline mileage,abnormal wear patterns, dramatically reduced tire life, and poor safetyperformance. However, most users persistently demonstrate anunwillingness to monitor and maintain proper tire inflation, typicallywith the assistance with a filling station pressurized air supply orpump.

Alternative techniques to the filling station air pump for monitoringand adjusting tire pressure are known. For example, U.S. Pat. No.4,924,926 to Schultz et al. discloses a central tire inflation systemhaving a central pressure distribution and monitoring system whichallows tire pressure to be monitored and changed using dashboardcontrols. Inadequacies of this system include difficulty in sealingjoints between the central pressure distribution and the rotating tirerim and tire. Additionally, the Schultz system requires significantinvestment in vehicle mounted equipment, and it is not easily used withconventional tire mountings or rims.

Another inflation system is disclosed by U.S. Pat. No. 5,119,856 toZarotti, wherein tire pressure is adjusted with an inflatable bladderinternal to the tire, the bladder containing a two-phase materialresponsive to a thermoelectric heater/cooler. A pressure sensor monitorstire inflation pressure while power is delivered to the tire through anelectromagnetic coupling mounted partially on the stationary portion ofa car and partially on each tire. Control information is deliveredthrough a modulated carrier transmitted through the electromagneticcoupling, and is locally compared to the monitored tire pressure.Problems with the Zarotti device include the requirement to maintaintire inflation at an adequate level prior to the servo startup,relatively slow adjustment to operating pressure level followingstartup, and the requirement for vehicle modification to accommodate thesystem.

U.S. Pat. No. 4,922,984 to Dosjoub et al., discloses entirely mechanicalmeans for automatically maintaining proper tire inflation. However, thereliance on mechanical techniques yields high cost and low reliability.

U.S. Pat. No. 4,349,064 to Booth discloses means for pumping air into atire by using changes in centrifugal force pursuant to changes invehicle speed. As the centrifugal force increases, a weight compresses aspring, and this motion is converted into pumping action. Again, thereliance on mechanical techniques yields high cost and low reliability.

U.S. Pat. No. 4,651,792 to Taylor pumps air by using changes in thecentrifugal force as seen by a tread-mounted device during each rotationof the tire. As the portion of the tire with the tread-mounted devicecomes in contact with the ground, that portion undergoes deformationwhich changes the effective radius of the tire, thus changing thecentrifugal force. As the centrifugal force increases, a weightcompresses a spring, and this motion is converted into pumping action.Again, the reliance on mechanical techniques yields high cost and lowreliability.

It would therefore be desirable to pump a gas between a plenum, such asa tire, and a point exterior to the plenum, without the power, size,weight, and cost of pump assemblies presently available.

SUMMARY OF THE INVENTION

In surmounting the foregoing disadvantages, the present inventionprovides apparatus for establishing and/or maintaining a predeterminedgas pressure within a tire or other plenum.

One embodiment of the invention is a gas transfer system for maintainingthe pressure within a plenum that includes a power source, a pressuresensor, a control unit, and a gas transfer mechanism. The gas transfermechanism can include a micromechanical device, comprising one or morepumping units that transfer gas from one pressure zone to another.

Another embodiment of the invention is a self-adjusting tire inflationsystem including a vehicle independent power source, such a photovoltaiccell, a thermocouple, or a deformable PVDF film. The gas transfermechanism can include a micromechanical device, comprising one or morepumping units that pump external air into a vehicle tire, by raising thepressure of the air incrementally. Either electromagnetic valvesresponsive to signals from a control unit, or passively biased valvesresponsive to gas pressure, control the flow of gas through the gastransfer mechanism.

One embodiment of the micromechanical device includes a heatable andsealable chamber, wherein gas is introduced to the chamber, the chamberis sealed and then heated to cause the pressure of the gas to increase,whereupon the pressurized gas is released directly into the plenum oranother chamber leading to the plenum for further elevating the gaspressure.

In another embodiment of the micromechanical device, an air transferchamber and a pumping chamber are separated by a flexible membrane,wherein compression of the plenum provides a pumping force. Valves areprovided in the micromechanical pumps to regulate movement of the gasthrough the pumps.

The valves of the various embodiments of the micromechanical devices caninclude a deformable element that is urged against or away from asealable surface by electrical attraction or repulsion in response to asignal from a control unit. In other embodiments, the valves are biasedagainst a sealing surface and are responsive to gas pressure.Additionally, the system components can either be discrete componentsthat may be spatially dispersed, or one or more components may comprisean integral device.

Still other aspects of the invention include a method of pumping a gas,and devices and methods for generating power for a micromechanical pump.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and the attendantadvantages and features thereof will be more readily understood byreference to the following detailed description when considered inconjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram of a gas transfer system of the invention;

FIG. 1A is a block diagram for a non-contact, signal transfer betweenthe control unit and the input/output devices shown in FIG. 1;

FIG. 2 is a schematic diagram of an implementation of a gas transferdevice of the gas transfer system of FIG. 1;

FIG. 3 is a schematic diagram of an alternative implementation of thegas transfer device illustrated in FIG. 2;

FIGS. 4-7 are a series of schematic illustrations depicting stages ofoperation of another embodiment of the gas transfer system of FIG. 1;

FIGS. 8A and 8B are a schematic representation of an embodiment of apower source of the gas transfer system of FIG. 1;

FIG. 9 is a schematic representation of another embodiment of a powersource of the gas transfer system of FIG. 1;

FIG. 10 is a schematic illustration of an integrated assembly embodimentof a gas transfer system of the invention;

FIG. 11 is plan view of the gas transfer mechanism of the system of FIG.10;

FIG. 12 is a sectional view of the gas transfer mechanism of FIG. 11;

FIG. 13 is a plan view of a lower portion of the gas transfer mechanismof FIG. 11;

FIG. 14 is a sectional view of the pressure sensor of the gas transfersystem of FIG. 10; and

FIG. 15 is a plan view of an internal portion of the pressure sensor ofFIG. 14.

DETAILED DESCRIPTION

FIG. 1 is a block diagram for a gas transfer system 10 having four majorcomponents, including a power source 12, a pressure sensor 14, a controlunit 16, and gas transfer mechanism 18. In the description whichfollows, an embodiment of the system is described that is associatedwith a plenum 20, such as a vehicle tire or one or more bladders withina shoe.

Generally, the power source 12 provides power to the pressure sensor 14,the control unit 16, and the gas transfer mechanism 18. The gas transfersystem 10 includes one or more gas transfer apertures 22 or passagesadapted to conduct a gas between the apertured plenum 20 isolating afirst environment from a second environment.

The pressure sensor 14 senses the pressure of a gas within the plenum 20using techniques known to those skilled in the art. In one embodiment,the pressure sensor 14 is a microelectromechanical (MEMS) structure,similar to commercially available semiconductor pressure sensors, andincludes an air inlet 24 into the sensor 14. Although the systemcomponents are illustrated as separate structures, they can beintegrated as a single structure.

The pressure sensor 14 provides a signal 26 representing a currentpressure value or a pressure change to the control unit 16, which caninclude a simple microprocessor implemented in a low power statictechnology, such as CMOS. In one embodiment of the system 10, thecontrol unit 16 stores a digitally encoded target pressure value, whichcan be loaded at the time of manufacture or as required by a system userthough an input device 28 located proximate the system 10, such as onthe tire or valve stem, or from a remote input device in communicationwith the control unit 16, such as in a vehicle cab accessible from thedriver's seat. Additionally, an output device such as display a 30 canbe provided to monitor system status and pressure values. FIG. 1illustrates an input/display unit 32 in communication with the system 10via signal paths 34, 36.

When the plenum 20 is a rotating vehicle tire, several challenges arepresented with respect to signal passage between a remote input/displayunit 32 and the control unit 16. FIG. 1A illustrates that this challengecan be overcome by providing "non-contact" signal transmission betweenthe control unit 16 and the input/display unit 32. In this embodiment,an emitter 33 in communication with the control unit 16 transmits asignal to a receiver 35 in communication with the input/display unit 32from the control unit 16. A similar but opposite paring of a receiver33' and an emitter 35' transmits a signal from the input/display unit 32to the control unit 16.

The emitter/receiver pairs can be adapted to function with forms ofelectromagnetic radiation, as is known in the art, such as through apair of electromagnets or capacitor plates, one on the vehicle and oneon the wheel. Other forms of communication between emitter/receiverpairs include radio communication or modulated light communication,mediated by a light-emitting-diode and photocell pair. Alternatively,acoustic communication, such as ultrasonic, can be provided by aspeaker-microphone combination.

In another embodiment of the system 10 directed to vehicle tires, atarget tire pressure is determined by a vehicle computer that modifiesthe target pressure as a function of the current weight of the vehicle,the current weather, the habits of the current driver, or otherconditions, such as geographic location or road conditions at aparticular geographic location. The condition information can becommunicated to the vehicle computer via cellular telephone, globalpositioning satellites, roadside visual displays, or other means.

Additionally, the control unit 16 can be configured so that a loss ofpower from the power source 12 does not have an adverse impact onpressure control. In one embodiment of the system 10, failure of thepower source 12 or power transmission to one or more of the other systemcomponents causes the gas transfer mechanism 18 to stop functioning,thus preventing gas from being introduced or exhausted from the plenum20.

The gas transfer mechanism 18 can change the amount of gas in a plenum20, not by changing the effective volume of the gas, but by increasingthe pressure in the plenum 20 in response to a control signal 38. Thus,pressure adjustment involves adding gas or removing gas. Typically,removing gas is easier than adding gas, since a pressurized plenum 20,such as a tire, is at a higher pressure than atmospheric pressure, andsimply opening a valve 40 releases pressure. Adding gas to the plenum 20is more challenging, especially with a very small, light-weight,low-power device.

FIG. 2 illustrates a micromechanical pump 42 that is an exemplaryembodiment of the gas transfer mechanism 18 of the system 10 that meetsthe aforementioned challenge. The exemplary micromechanical pump 42 isshown having a first and a second pumping unit, 44 and 46, respectively.However, a single pumping unit is adequate for certain applications,depending on the pressure differential which must be maintained orestablished, and the power available. More pumping units can beincorporated into the micromechanical pump 42 as required. The term"micromechanical" as used herein includes devices that are strictlymechanical, electromechanical, pneumatic, or a combination thereof. Withrespect to device size, the term "micromechanical" encompasses nanoscaleand lithographically fabricated implementations, as well as largerdevices, such as systems the size of a typical wrist watch body orlarger.

Each pumping unit 44 and 46 includes a chamber 48 and 50, respectively,apertured to permit a gas to flow though the pumping unit 42. Generally,for a micromechanical pump 42 having two pumping units, air atatmospheric pressure enters the first chamber 48 and is pumped into thesecond chamber 50, from which it is pumped out of the micromechanicalpump into the plenum 20, such as a tire, which retains a volume of airat greater than atmospheric pressure.

In the illustrated embodiment of FIG. 2, the micromechanical pump 42 isshown in a sealed relationship with a tire wall 54 which defines theplenum 20, and the tire wall 54 having an opening 56 therethrough topermit passage of air to and from the micromechanical pump 42. An airfilter 58 is provided at the face of a first aperture 60 or air inlet.Owing to the very small scale of the micromechanical pump 42 and thecomponents thereof, the filter 58 is especially important forapplications wherein the gas presented to the micromechanical pump 42 iscontaminated with particulate matter.

The first aperture 60 is provided with a first valve 62 movable betweena first or open position and a second or closed position which seals theaperture 60. In the exemplary embodiment, the aperture 60 is defined bya first valve seat 64, and the first valve 62 includes a deformableinsulator 66 and a deformable conductor 68 having a surface ofsufficient size and impermeability to engage the valve seat 64 in asealing relationship and thereby block the aperture 60. A secondaperture 70 in the first chamber 48 is defined by a second valve seat 72having associated therewith a second valve 74 like the first valve 62.

The second pumping unit shares the second valve assembly 74 with thefirst pumping unit 44 and it defines a first aperture 70 in the secondchamber 50. A second aperture 74 for the second chamber 50 is defined bya third valve seat 76 having associated therewith a third valve 78 likethe first and second valves, 62 and 74, respectively. The first andsecond chambers, 48 and 50, are adapted to be heated and can include anintegral heat source. In the illustrated embodiment, a resistive element79 is provided within each chamber.

Power to heat the resistive elements 79 is provided via wires 80connecting the resistive elements to the power source 12. The powersource 12 further provides power to each of the valves 62, 74, and 78,wherein a charge of a first potential is provided to the deformableconductor 68 via a first wire 80, and a charge of a second potential isprovided to the valve seat 64 via a second wire 82.

The micromechanical pump 42 of FIG. 2 operates over one or more pumpingcycles as follows. Air enters the opening 56 in the plenum wall 54 andis cleaned by the air filter 58 as it passes through the first aperture60 and into the first chamber 48. In response to a signal from thecontrol unit 38, the power source 12 energizes the deformable conductor68 and the valve seat 64. The difference in potential between thedeformable conductor 68 and the first valve seat 64 creates an electricfield which causes the deformable conductor 68 to be attracted to thefirst valve seat 64, thus closing the first valve 62.

The control unit 16 also commands current to be passed through theresistive element 79 to be energized, thus heating it and a volume ofgas 83 within the first chamber 48. As the gas 83 is heated, it expandsand a portion thereof is forced to move through the second valve 74which is de-energized, or forced open despite being energized, into thesecond chamber 50. At a predetermined moment, based upon the time ittakes the resistive element 79 to heat the gas 83 to a predeterminedtemperature, the control unit 16 interrupts the current flow to theresistive element 79, commands the second valve 74 closed and the firstvalve 62 open. At this moment, the gas pressure in the first chamber 48is below that of the gas pressure exterior to the plenum 20, because theheating cycle has driven much of the gas 83 into the second chamber 50.When the remaining gas in the first chamber 48 cools, its pressure dropsto and below the pressure of gas exterior to the plenum 20, allowingmore gas to flow in through the first valve 62, wherein the pumpingcycle may be repeated.

The second pumping unit 46 operates in the same manner as the firstpumping unit 44 to move the pressurized gas 84, transferred from thefirst pumping unit 44, into the second chamber 50 at a still higherpressure toward the interior 86 of the plenum 20. Depending on theselected pressure elevation, two or more cycles of the first or lowerpressure pumping unit 44 may be required for each cycle of the second orhigher pressure pumping unit 46. As stated hereinabove, the heating of achamber may be provided by other devices known to those skilled in theart.

FIG. 3 illustrates another embodiment of the gas transfer mechanism 18,including a micromechanical pump 88 having an alternative implementationof valves. Whereas, the valves 62, 74, and 78, described with respect toFIG. 2 are active, requiring input from the control unit 16 and powerfrom the power source 12, the valves of FIG. 3 are passive, requiringneither power or command inputs. Each valve 90, 92, 94 includes asealing surface 96 biased against a valve seat 98 to block the aperture100 defined by the valve seat 98. In the exemplary embodiment of FIG. 3,each of the valves is shown in its biased or closed state. Each valve ispiston shaped, having a helical spring 102 disposed around a shaft 104,the spring 102 being compressible between a rear portion of the sealingsurface 98 and a spring support, such as another valve seat. Otherbiasing means known in the art are also suitable. An air filter 106 isprovided between the opening 56 in the plenum wall 54 and the aperture100 leading to a first pumping unit 107, provided with a resistiveelement 108 adapted for electrical connection with a power source 12 viawires 110. A second pumping unit 112 is provided, as required, and issimilar in structure to the first pumping unit 107.

The micromechanical pump 88 of FIG. 3 operates in the following manner.The control unit 16 commands current to pass through the resistiveelement 108 of the first pumping unit 107, thus heating the resistiveelement and the gas within the chamber 106. As the gas is heated, itspressure rises until it is sufficient to overcome the biasing force ofthe spring 102 of the second valve 92, thus forcing the second valve 92to open and forcing a portion of the gas through the second valve 92. Ata predetermined moment, the control unit 16 interrupts the current flowto the resistive element 108. At this instant, the pressure in firstchamber 106 drops below that of the outside gas, allowing higherpressure gas external to the plenum 20 to overcome the biasing force ofthe spring 102 in the first valve 90.

The second pumping unit 112 operates in the same manner as the first tomove the first volume of gas at still higher pressure toward theinterior 86 of the plenum 20. Depending on the selected pressureelevation, two or more cycles of the first or lower pressure pumpingunit 107 may be required for each cycle of the second or higher pressurepumping unit 112. As with the embodiment of FIG. 2, the second valve 92is shared by both of the pumping units.

FIGS. 4-7 illustrate schematically another embodiment of themicromechanical pump 114, adapted to the special conditions of aflexible plenum 20 such as a vehicle tire or shoe insole. Thisembodiment uses temporary overpressure caused by compression of theplenum 20. In the case of a tire, the compression occurs as the tirerolls over bumps, or when the vehicle accelerates or decelerates.

The micromechanical pump 114 includes a pumping chamber 116; an airtransfer chamber 118; a first valve 120 between the air transfer chamber118 and the exterior of the plenum 20; a second valve 122 between theair transfer chamber 118 and the gas within the plenum; a third valve124 between the pumping chamber 116 and the gas within the plenum; and aflexible membrane 126 separating the air pumping chamber 116 from theair transfer chamber 118.

FIG. 4 illustrates the micromechanical pump 114 in a static state,wherein the pressure within the plenum 20 is constant. Thus, thepressure within the plenum 20 is equivalent to the pressure within thepumping chamber 116 and the air transfer chamber 118. Because thispressure is higher than the pressure exterior to the plenum, the firstvalve 120 remains closed, preventing the escape of air from the plenum20.

Referring to FIG. 5, the plenum 20 is shown during an overpressurecondition, causing the third valve 124 to open, and allowing higherpressure air to flow into the pumping chamber 116. The higher pressureair in the pumping chamber 116 forces the flexible membrane 126 toprotrude into the air transfer chamber 118, thereby elevating thepressure of the air in the air transfer chamber 118 as well.

As the overpressure drops, as illustrated in FIG. 6, the pressure in thepumping chamber 116 remains high because the third valve 124 closes toprevent air from flowing back into the interior of the plenum 20. Thishigher pressure maintains the higher pressure in the air transferchamber 118 and that higher pressure forces air from the air transferchamber 118 through the second valve 122 into the plenum 20. Once theplenum 20 returns to a state of constant pressure, the air in the airtransfer chamber 118 is exhausted so that no further air flows throughthe second valve 122, which closes. The higher pressure air in thepumping chamber 116 bleeds back into the inside of the plenum 20 througha small orifice 128 in the pumping chamber. Alternatively, the orifice128 can be incorporated into the third valve 124.

As shown in FIG. 7, air bleeding back into the plenum 20 allows theflexible membrane 126 to spring or rebound back to its static position,thus drawing air into the air transfer chamber 118 from the exterior,thus returning the micromechanical pump 114 to the state depicted inFIG. 4.

With respect to the valves, either passive, biased valves 120, 122, and124, such as those described with respect to FIG. 3 can be used, as wellas the electrically activated valves described with respect to FIG. 2.

Heretofore, the power source 12 for the exemplary micromechanical pumps42, 88, and 114 has been described in general terms. With respect to anembodiment of a micromechanical pump that controls pressure in a cartire, the energy required from the power source for a typical automotivetire volume, V, of 10 liters can be estimated in the following manner.An estimate of a typical tire pressure loss rate is 1 PSI/week, or 6894Nt/m/m per week. One week is 7×24×60×60=630,000 seconds. The typicaltire is inflated to about 30 PSI. The energy lost in leaking air, dE,(assuming nearly constant volume) is VdP, or (10 liters) (6900Nt/m/m)=69 joules. This energy is lost during an interval of 630,000seconds, giving a power loss of 110 microwatts. At a minimum, this isthe theoretical average power input to the system 10 required tomaintain tire pressure with the specified leak rate. In practice,substantially more power is required due to intrinsic and accidentalinefficiencies in the gas transfer mechanism, the control unit 16, andthe pressure sensor 14.

Although power sources known to those skilled in the art, such as abattery, can provide the required power, applications for vehicle tiresbenefit from a renewable power source that has few or no mechanical,pneumatic, hydraulic, thermal, or electrical couplings between therotating rim and tire and the remainder of the vehicle.

One embodiment of the power source 12 which meets these requirements isa photovoltaic cell illuminated by ambient light. Sunlight suppliesapproximately 1 KW/m/m. A conversion efficiency of 10% is achievablewith simple solar cells. If only 1% of the direct sunlight is available,due to shading, cloud cover, and night, an available potential powerdensity of 1 Watt/m/m is obtainable. Because as much as 1 mW could berequired to power the system, a light collection area of approximately 3cm×3 cm is required. The photovoltaic cell can be positioned on the tiresidewall.

FIGS. 8A and 8B illustrate an embodiment of the power source 12, whereina piezoelectric material, preferably poled orientedpolyvinylidenefluoride (PVDF, tradename Kynar) plastic sheet or film130, is bonded (such as during ply layup) under a tire tread 132 betweenan inner layer 134 of a tire tread 132 and an outer layer 136. The PVDFfilm 130 generates an electric charge as it deforms with the tire tread,the tread deforming at a location 138 where the tread 132 comes intocontact with the ground or a road 140.

Another embodiment of the power source 12 takes advantage of atemperature differential between portions of a tire/rim assembly 142 topower a thermocouple or other temperature-to-energy conversion devices.FIG. 9 illustrates a power source wherein a temperature differentialbetween hot and cold junctions represents a thermocouple 144 acrosswhich is generated a potential related to the heat differential betweenthe junctions. First and second wires, 146 and 148, respectively deliverelectrical energy to a gas transfer mechanism 18, and a third wire 150connects a portion of the first wire 146 to a portion of the second wire148. The first and second wires comprise one material such as copper,whereas the third wire comprises another material, such as constantan.The junction between the first and third wire is a hot junction 152located at a point in or near the tread 154. The junction between thesecond and third wire is a cold junction 156 located at a point in ornear the tire sidewall 158. Alternatively, the cold junction 156 can besituated in or near a portion of the wheel rim 142.

Thus, when a power source 12, such as that described with respect toFIGS. 8 and 9 is associated with a micromechanical pump of FIGS. 2, 3,4, and 5, an integrated self inflating tire system 10 is provided. Forthis implementation, the pressure sensor 14, power source 12, gastransfer mechanism 18, and control unit 16 are integrated on a singlesilicon substrate, such as is described hereinbelow with respect toFIGS. 10-15. The system can thus operate to either add or subtract airfrom a tire interior, depending upon the desired direction of air flow.Alternatively, air can be pumped into the tire by the system 10 withoutregard to the pressure within the tire, wherein a pressure relief valveperforms the necessary regulation. The system can be built into thetread or sidewall of a tire during layup or as part of the tire rimassembly.

Alternatively, the pressure sensor 14, gas transfer mechanism 18, andpower source 12 can comprise a device sized to take the place occupiedby an ordinary valve stem used for filling tires from external airpumps.

FIG. 10 illustrates schematically an embodiment of a gas transfer system160, having an integral structure, connectable to a power supply 162.The system 160 includes a gas transfer mechanism 164, a control unit166, and a pressure sensor 168. The system 160 further includes firstair passages 170 and 172, and second air passages 174 and 176,associated with the gas transfer mechanism 164 and the pressure sensor168, respectively.

FIGS. 11 and 12 are plan and sectional views, respectively, of anexemplary embodiment of the gas transfer mechanism 164, including afirst portion 178 and a second portion 180. An interior facing surfaceof the second portion 180 is shown in plan view in FIG. 13. The firstair passage 170 and the second air passage 174 lead to a first valve 182and a second valve 184, respectively, which control the passage of a gasto and from a heating chamber 186. Each valve 182 and 184 includes avalve seat 188 to which a first charge of a selected polarity can beapplied, and a movable sealing element 190, such as a flexible membranehaving a metallic plate 191 to which a second charge of a selectedpolarity can be applied via a wire 192. The second portion 180 furtherincludes a heating element 194 connected to wires 196 through which anelectric current can be passed to heat gas in the heating chamber 186.

The gas transfer mechanism functions in the following manner to pump airinto a plenum such as a tire, for example. A pumping cycle begins byapplying an electric current to the wire 192 of the movable sealingelement 190 associated with the first valve 182, imparting a charge tothe metallic plate 191, and causing it to be drawn toward the valve seat188 and held thereto in a sealing relationship, thus preventing gas frommoving through the first valve 182. Subsequently, the second valve 184is closed in like manner.

After the second valve 184 is closed, the first valve 182 is opened byremoving the charge on the metallic plate 191. An electric current ispassed through the heating element 194, causing gas in the heatingchamber 186 to expand, forcing it through the first valve 182 and intothe first air passage 170. The first valve 182 is then closed and thesecond valve 184 reopened to continue the pumping cycle if desired.

Gas can be removed from a plenum, assuming the inside pressure is higherthan the outside pressure, by simply placing both valves 182 and 184 intheir open state. Alternatively, the gas inside the plenum can be pumpedout of the plenum by reversing the opening and closing of the valves 182and 184 in relation to the heating cycle.

Although the unelectrified state of the valves 182 and 184 is "open," inan alternative embodiment of the gas transfer mechanism 164, theunelectrified state is "closed." This configuration is provided byimbuing the flexible sealing element 190 of each valve with a shape thatplaces them in sealing contact with the valve seat 188, until arepulsive charge is placed on the metallic plate 191.

FIG. 14 is a sectional view of an exemplary pressure sensor 168 for thegas transfer system 160, wherein the first air passage 172 associatedwith the pressure sensor leads to a first chamber 198 and the second airpassage 176 leads to a second chamber 200, separated from the firstchamber by a deformable element 202. The deformable element 202 deformsin proportion to the pressure difference between gas in the first andsecond chambers 198 and 200, respectively.

FIG. 15 is a plan view of a central layer of material 204 of thepressure sensor 168, having a resistive element 206 positioned on thedeformable element 202. The resistive element 206 can include apiezoelectric material or other material having a resistance that varieswith deformation. The deformation of the resistive element 206 ismeasured by passing a current through wires 208 connected to theresistive element 206.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. A variety of modifications and variations arepossible in light of the above teachings without departing from thescope and spirit of the invention, which is limited only by thefollowing claims.

What is claimed is:
 1. A tire pressure maintenance system, comprising:apressure sensor for measuring gas pressure in said tire and providing apressure signal representative of said gas pressure; a control deviceresponsive to said pressure signal and to provide a control signal; anda gas transfer mechanism including a micromechanical pump responsive tosaid control signal for transferring gas into said tire from a locationexternal to said tire, said micromechanical pump including,a pumpingchamber; an air transfer chamber; a flexible membrane separating saidair pumping chamber from said air transfer chamber; a first valvebetween said air transfer chamber and a location external to said tire;a second valve between said air transfer chamber and a location internalto said tire; and a third valve between said pumping chamber and alocation internal to said tire.
 2. The tire maintenance system of claim1, wherein said pumping chamber includes a means for bleedingpressurized gas from said pumping chamber to an internal region of saidtire.
 3. The tire maintenance system of claim 1, wherein said firstvalve, said second valve, and said third valve are each passively biasedand responsive to gas pressure;said first valve opening in response toan excess of gas pressure external to said tire with respect to the gaspressure within said air transfer chamber; said second valve opening inresponse to an excess of gas pressure within said air transfer chamberwith respect to the gas pressure within said tire; and said third valveopening in response to an excess of gas pressure within said tire withrespect to the gas pressure within said pumping chamber.
 4. The tiremaintenance system of claim 1, wherein the flexible membrane deforms inresponse to a pressure change in said tire.
 5. The tire maintenancesystem of claim 1, wherein a pressure increase within said tire causesthe pressure within said pumping chamber to be increased, therebydeforming said flexible membrane.
 6. The tire maintenance system ofclaim 5, wherein deformation of said flexible membrane causes a volumeof said pumping chamber to increase and a volume of said transferchamber to decrease.